Physicochemical Characterization of Sludge Originatingfrom Vegetable Oil–Based Cutting Fluids

A. J. B. Sutton1 and D. J. Sapsford2

Abstract: Vegetable oil–based cutting fluids are a relatively recent development in large-scale metal machining. A metal machining factoryin Wales that switched from mineral oil-based to vegetable oil-based cutting fluids has experienced the occurrence of a problematic floatingsludge within the settling and holding tanks at the on-site effluent treatment plant. Physicochemical analyses have found that the sludge iscomposed of on average 33% water, 20% oleic acid, and 18% palmitic acid, originating from the vegetable oil–based cutting fluids used at thefactory. A solvent separation step was devised and used successfully to separate water inherent within the sludge so as to study the division ofthe inorganic elements within the water and organic phases of the sludge. It was found that only a minor constituent of the sludge can beaccounted for by Ca-bonded fatty acids. Formation of the sludge is suspected to be due to the biologically induced hydrolysis and oxidation ofthe oils from esters to the free fatty acids and subsequent creaming, forming a layer of stable floating sludge on the surface of the effluentstorage tanks. DOI: 10.1061/(ASCE)EE.1943-7870.0001014. This work is made available under the terms of the Creative Commons Attri-bution 4.0 International license, http://creativecommons.org/licenses/by/4.0/.

Author keywords: Fat, oil, and grease (FOG); Sludge; Esters; Rapeseed; Oleic acid; Palmitic acid; Tribology.

Introduction

This study focuses on the occurrence of a problematic floatingsludge that accumulated within the holding tanks of an on-siteeffluent treatment plant (ETP) at a metal-machining plant. Thesludge occurrence began after the introduction of vegetable oil–based cutting fluid (VOCF) in the plant to replace the use of mineraloil–based cutting fluid (MOCF). The sludge occurrence was notlimited to a single on-site treatment plant, but has been reportedto be occurring at many other plants that have likewise switchedfrom MOCFs to VOCFs for their machining processes.

The use of vegetable oils as a replacement for mineral oils inlubrication applications (such as VOCFs) becoming much morewidespread (Bartz 1998; Erhan and Asadauskas 2000) has beenbrought about by the need for environmentally friendly and biode-gradable products. In common with other lubricants, MOCFsand VOCFs are known within the tribology field to degrade(e.g., Rasberger 1997; Schneider 2006) and form sludge, and spe-cific tests exist to measure the likelihood of oil to oxidize or hydro-lyze and form sludge [e.g., ASTM 2015; Coordinating EuropeanCouncil (CEC) L-103-12 (CEC 2014); Wooton 2007]. Sensitivityto hydrolysis, low resistance to oxidative degradation, and poorlow-temperature properties are recognized as major issues forvegetable oils (Erhan and Asadauskas 2000; Schneider 2006).However due to the recent (circa 2000) introduction of VOCFsto the metal-machining industry and limited introduction at manu-facturing plants, the large-scale sludge accumulation problem inETPs reported in this paper has hitherto not been systematically

recorded or studied and is thus of interest to environmentalengineers.

A similar sludge was found after spillage of VOCFs duringshipping at sea (ITOPF 2010); aside from this there is currentlyno literature available on VOCF sludge and its composition. A re-lated phenomenon, fat, oil, and grease (FOG) formation in sewers,has been more extensively studied (e.g., Nitayapat and Chitprasert2014; He et al. 2011; Williams et al. 2012; Keener et al. 2008) andresults in sludge with similar physiochemical characteristics to thatobserved in the ETP. FOG is partially composed of metallic salts offatty acids and free fatty acids (FFAs) (He et al. 2011; Williamset al. 2012), with a water content of 15-95% (Williams et al.2012). It has similar semisolid, adherent, and buoyant physicalproperties as the sludge found at the metal-machining factories.The main source of FOG in sewers is from cooking oils, withone of the most widely used being rapeseed oil (USDA 2009).

The aims of this study are to present a methodology for thephysicochemical characterization of the VOCF sludge, determinewhether soap constitutes a significant fraction of the sludge (as hasbeen found in studies of FOG), and suggest a plausible mechanismto the route of formation of the VOCF sludge.

Vegetable Oil–Based Cutting Fluids

Cutting fluids are used in the machining and forming of metals tolubricate and cool the processes. Several kinds of cutting fluidexist including neat oils, oil in water emulsions using petroleumdistillates (MOCFs) or vegetable oils (VOCFs), pastes, aerosols,and gases. The use of VOCFs has increased since its introductionin circa 2000 because of their demonstrably reduced environmentalimpacts and toxicity compared to MOCFs (Lawal 2012; Clarens2008; McManus 2003). Commonly used vegetable oils used in-clude soybean, sunflower, rapeseed, and palm oil (Shashidharaand Jayaram 2010). All vegetable oils comprise numerous compo-nents such as monoglycerides, diglycerides, and FFAs in varyingconcentrations; however their main constituent is triglycerides,with three fatty acid chains linked by a glucose molecule

1Postgraduate Student, Cardiff School of Engineering, Cardiff Univ.,Parade, Cardiff CF24 3AA, U.K. (corresponding author). E-mail:suttona3@cardiff.ac.uk

2Senior Lecturer, Cardiff School of Engineering, Cardiff Univ., Parade,Cardiff CF24 3AA, U.K.

Note. This manuscript was submitted on November 28, 2014; approvedon June 29, 2015; published online on September 4, 2015. Discussionperiod open until February 4, 2016; separate discussions must be submittedfor individual papers. This paper is part of the Journal of EnvironmentalEngineering, © ASCE, ISSN 0733-9372/04015057(8)/$25.00.

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(Ştefanescu et al. 2002), e.g., trimethylolpropane trioleate. Thephysical properties of the oils are influenced by which FFAs arebonded to the glucose (or alternative) molecule, which may notall be the same fatty acid (Shashidhara and Jayaram 2010). Forexample, the triglycerides in rapeseed oil are composed of oleicacid (60%), palmitic acid (4%), linoleic acid (20%), and alpha-linoleic acid (10%); the values in parentheses indicate the averagepercentage composition of the ester by the named free fatty acid(Gunstone 2009).

Site Description

A floating sludge of unknown composition had accumulated withinthe settling and holding tanks at an on-site ETP in a metal-machining factory in Wales. Prior to 2006, the factory had beenusing MOCFs; however, following a drive to reduce the environ-mental impacts of its cutting processes, it changed the bulk of itssystems to use VOCFs and vegetable oil–based hydraulic fluids(lubricants). Within 6 months a floating sludge had started to formin the on-site effluent settling tanks. Other sites with similar sludgeoccurrence had also recently changed from MOCFs to VOCFs.

The VOCFs used as cutting fluid, slideway oils, and hydraulicfluids at the plant are refined rapeseed oil and syntheticallyproduced oil using oleic acid as the FFA. Triethanolamine is thesurfactant used. Cutting fluid is continuously circulated aroundthe metal-machining terminals, with each circuit being toppedup with water (lost to evaporation), cutting oil, and surfactant. Bio-cides and acticides are also added to increase the longevity of thecirculating VOCF. Along with these chemicals the systems usetrace amounts of antifoam, pH adjusters, and corrosion inhibitors.When the VOCF quality is compromised the circuit is completelydrained to the ETP holding tanks. Other inputs to the ETP includesurfactant-water mixtures used in cleaning the components.

Approximately 350 m3 ofVOCFwas used annually at the factory,with the bulk of this being treated at the on-site ETP. The ETP usesceramic ultrafiltrationmodules,which producewaterwith a chemical-oxygen demand averaging 5,000 mg=L that is disposed of into thesewer. The waste oil produced is approximately 50% purity and istransported byvacuum tanker off-site for acid splitting or incineration.TheETP treats approximately24,000 m3 ofwastewater annually. Theeffluent is stored in 500-m3 settling tanks for up to a month beforebeing processed through the ultrafiltration modules. It was in these

settling and holding tanks that the floating sludgewas occurring. Overa period of 6 years there has been an estimated 1,000 m3 of sludgeformed between the five 500-m3 effluent settling tanks. Two types ofsludgewere observed (Fig. 1). Sludge 1 is firmer, higher viscosity, andmore claylike in appearance, while Sludge 2 is comparatively lower inviscosity, but still semisolid.

Methods and Materials

Samples were extracted from the holding and settling tanks andwere taken at a depth of 0.1–0.3 m below the surface in multiplelocations. The sludge was mixed by hand to homogenize itand subjected to a range of physicochemical analyses to determineorganic and inorganic composition, including microscopicexamination, gas chromatography–mass spectrometry (GC-MS),inductively coupled plasma-optical emission spectrometry (ICP-OES), total inorganic-organic carbon determinations, Fouriertransform infrared spectroscopy (FTIR), and melting point determi-nation. A technique for the separation of the water and organics wasdevised to differentiate aqueous inorganic elements in the sludgewater from inorganic elements associated with the sludge organicfraction. The methods detailed subsequently are summarized in theflow diagram in Fig. 2, which shows the processes that are under-taken on the sludge prior to each analysis, with each processstarting with homogenizing the sludge. For example, sludge thathas been analyzed by GC-MS was initially homogenized, followedby water separation, then accelerated solvent extraction (ASE),filtration, and finally analysis by GC-MS. Inorganic analytes inthe water and solvent extract filtrate were calculated from ICP-OES measurements on other parts of the sludge. FTIR analysiswas conducted on the unadulterated sludge.

Optical Microscopy

A fresh (same day sampled) homogenized sample of sludge wasused for analysis for all of these tests. A sample was placed ona microscope slide and carefully compressed using another slide.Magnifications between 10× and 200× were used to observe thesample, using dark field, light field, and phase contrast microscopy.Alongside this a dip slide was used to calculate the number of livingbacteria within the sample. Both sides of the dip slide were wipedacross the surface of the homogenized sludge and then incubated at37°C for 48 h.

Fig. 1. (a) Consistency of Sludge 1 held in an analyst’s hands; (b) Sludge 2 in one of the 500-m3 settling tanks at the on-site ETP in Wales (images byA. J. B. Sutton)

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Total Carbon, Total Organic Carbon, and TotalInorganic Carbon

Vacuum-dehydrated sludge was used for carbon content analysis.Total carbon (TC) and total inorganic carbon (TIC) analysiswas conducted using a Shimadzu SSM-5000 A solid samplemodule and Shimadzu TNM-1 detector; total organic carbon(TOC) was calculated by the difference between the two. TC intriplicate was measured using 0.5 g of dehydrated sludge.A calibration check was run and showed accuracy to be �5%against a glycerol standard for TC and against calcium carbonatestandard for TIC prior to analysis of the sludge. Three repetitions ofeach analysis were undertaken, with the results showing theaverage of the three, giving Sludge 1 a standard deviation of�0.6% and Sludge 2� 1.5%.

Fourier Transform Infrared Spectroscopy

AVarian 3100 FTIR was used along with a Varian 600 UMA FTIRmicroscope to analyze the unadulterated sludge, which was placeddirectly on the crystal. Scans at a resolution of 4 cm, wave numberof 4,000–650 cm−1, and 100 cycles were conducted.

Melting Point

Melting point tests have been used in analyses of FOG samples(e.g., Williams et al. 2012; Nitayapat and Chitprasert 2014). Themelting point test in this case was conducted to aid in differentia-tion between metal-bound fatty acids and FFAs. Using a syringe,10 g of sludge was injected into the bottom of a test tube and placedinto a water bath, which was heated at a rate of 5°C=min fromambient. Once the sludge started to melt the bath was allowedto cool by 10°C and the test tube replaced with fresh sludge.The heating rate was altered to 1°C=min increase, where the initialand full melting points were measured to determine the meltingrange. This was repeated three times for each sample.

Water Separation

Separation and removal of the water is required for organicsanalyses by GC-MS and has been achieved using various methodsin the literature applied to FOG samples and oily wastes and soils:(1) melting the lipid followed by and water-fat separation bycentrifugation or use of anhydrous sodium sulphate to absorbwater, or by drying at 103°C (Hamilton and Rossell 1986) or105°C (Nitayapat and Chitprasert 2014), (2) entrainment distilla-tion using the Dean and Stark method (Hamilton and Rossell1986), (3) direct oven drying at temperatures between 101 and105°C (e.g., Hamilton and Rossell 1986; Williams et al. 2012;He et al. 2011; Keener et al. 2008), (4) freeze drying, (5) microwavedrying [ASTM D4643-08 (ASTM 2008)], and (6) vacuum dehy-dration using a vacuum desiccator with silica gel crystals. Waterdetermination in oil sludge, but not water separation can beachieved using nuclear magnetic resonance (Zheng et al. 2013)or Karl-Fischer titration.

The properties of fats can be affected by excessive heat,chemical addition, and oxidation. In particular, heating can causeoxidation of fatty acids. Although drying is the most popularmethod, in this separation the aqueous inorganic elements remainwithin the dried sludge. Thus in this study liquid-liquid separationwas the preferred method to allow determination of the deportmentof inorganic elements between organic and water portions of thesludge.

Water separation was conducted for GC-MS and ICP-OESanalysis using the following technique. Into a separating funnel25 g of homogenized sludge and 50 mL of dichloromethane(DCM) were added and shaken vigorously, then left to settle for15 min to develop a boundary layer. The lower layer with DCMand dissolved organics was drained. This was repeated five times,at which point the DCM was clear, indicating that minimal solvent-soluble organics remained within the separating funnel. The solventwas evaporated from the solution, leaving the resultant oily residueto be weighed and subject to further analyses; this residue is

Fig. 2. Flow diagram detailing the separations undertaken and the analysis conducted on the sludge

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referred to as solvent-separated sludge (SSS).Three repetitionswere undertaken, with the results showing the average of the three.

Organics Extraction and GC-MS

Water-insoluble organics are typically extracted from environmen-tal solids, such as soils, clays, sediments, and sludge using Soxhletextraction [e.g., Furniss et al. 1989; U.S. EPA method 3545 (U.S.EPA 2007)]. In the present study the extractor used was a Dionexaccelerated solvent extractor (ASE100). A polar (hexane) andnonpolar (acetone) solvent mix, of 1∶1 ratio was used as the extrac-tant. The following ASE operating conditions were used: 175°Coven temperature, 10,300–13,800 kPa (1,500–2,000 psi) pressure,5–10 min static time, 60–75% of cell volume for flush, 60 s nitro-gen purge at 1,030 kPa (150 psi), and one cycle for extraction. (U.S.EPA 2007), repeated once to ensure complete extraction. Non-solvent-extractable (NSE) solids and solids >0.2 μm were filteredout during the extraction, leaving the ASE sludge for further process-ing. After extraction the solvent extract was cooled to room temper-ature, where solids were seen to have formed and were filtered usinga 0.2-μm filter. Solids filtered out at this point are hereafter referredto as solvent extract filter residue (SEFR), while the liquid to beanalyzed by GC-MS is known as the solvent extract filtrate.

The solvent extract filtrate was qualitatively and quantitativelyanalyzed using a Perkin Elmer Clarus 500 GC-MS using an Elite5MS column of 30 m length, 0.25 mm inner diameter, and filmthickness of 0.25 μm, with the following settings: system pressure72 kPa (∼10.5 psi), 1 mL=min helium carrier, injection tempera-ture of 250°C, and temperature program comprising 100°C initialtemperature, hold for 2 min, increasing at 4°C=min until 310°C,hold for 4 min. The column was cleaned between analysis runsusing blank DCM samples. Qualitative analysis was performed us-ing the NIST MS Search 2.0 database. Qualitative analysesindicated that palmitic and oleic acid were the dominant chemicals,with quantitative analyses being performed using a three-pointcalibration using standards made up from analytical grade palmiticacid and oleic acid in DCM. Triglycerides with more than 58 car-bon bonds in their structure were suspected to be in the sludge,however due to their high molecular mass and boiling point(e.g., 851°C for trimethylolpropane trioleate) they would not elutefrom the columns. Samples of 99.5% purity of palmitic and oleicacid were run using the same operating conditions to confirm thatthe NIST database was correctly identifying the compounds.

Microwave Acid Digestions and ICP-OES

Due to the organic content of the sludge (approximately 99%excluding water), digestion was required. Typical digestions inthe literature include the following examples used for FOG-typesamples: (1) refluxing with 50% HNO3 at 95°C (Williams et al.2012), (2) digestion using concentrated nitric acid at 95°C followedby addition of hydrogen peroxide (Keener et al. 2008; U.S. EPA1994), (3) microwave-assisted acid digestion using HNO3 andof HF (or other appropriate acid), KMnO4 or H2O2 used asoxidants (U.S.EPA 1996), and (4) ashing at 500°C followed by acid(HCl=HNO3) digestion (He et al. 2011).

Ashing was trialed but found to be problematic, thereforemicrowave digestion was used: To 0.5 g of sludge 9 mL ofHNO3, 3 mL of HCl, and 2 mL of H2O2 were added. The samplewas heated to 180°C in 5.5 min and maintained at that temperatureuntil the total heating time reached 10 min. The digested samplewas diluted to 50 mL using deionized water and subjectedto ICP-OES analyses (Perkin Elmer Optima 2100DV) using amultielement certified standard.

Ten repetitions of the full sludge extraction, microwave diges-tion, and ICP-OES analysis were conducted.

Results and Discussion

Optical Microscopy

Microscopic examination of wet sludge spread onto a glass sliderevealed that the sludge samples predominantly consisted of anemulsion of spherical liquid droplets, typical in appearance toan oil emulsion. A very small amount of microbial biomass,namely, some fungal fragments, were also present. Dip-slide testing(data not included) of the sludge revealed only moderate microbialgrowth. Fig. 3 shows spherical droplets of a liquid with anotherliquid. This is likely to be a water-in-oil emulsion (inverted emul-sion) due to the low water content of the sludge. Also visible areminor fragments of swarf from the machining processes.

Total Carbon, Total Inorganic Carbon, and TotalOrganic Carbon

The TC, TIC, and TOC analyses sludge showed that none of the TCpresent was present as TIC, ruling out the presence of dissolved orsolid phase carbonates in the sludge and that all the TC ¼ TOC forthe sludge. The values of TC for the vacuum dehydrated sludgewere 79.84% (�0.6%) and 75.47% (�1.5%) for Sludges 1 and2, respectively. TC corrected to the wet sludge was 55.60 and48.64% for Sludges 1 and 2, respectively.

Compounds Identified by GC-MS

A number of organic compounds were detected (Table 1) in thesolvent-separated sludge using the NIST library. Of these com-pounds palmitic acid and oleic acid had by far the largest peakareas, therefore these were subsequently analyzed quantitatively.No vegetable triglycerides were detected, although their presenceis suspected due to their existence in the virgin VOCF, but theGC-MS settings did not allow elution of triglycerides.

Fig. 3. Optical microscope image of Sludge 2 using transmitted lightfrom underneath at a zoom of 100×

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The quantitative analysis of the sludge revealed that palmiticacid formed 23.95 and 11.78% of total sludge mass and oleic acidcontent was 17.18 and 23.13% of Sludge 1 and Sludge 2,respectively. These are within the range of the 10–70% oleic acidcontent found in FOG (Nitayapat and Chitprasert 2014). Oleic acidand palmitic acid were found in many FOG samples (Nitayapat andChitprasert 2014; Keener et al. 2008; Williams et al. 2012) invarying concentrations and ratios.

Non-solvent-extractable solids account for 2.29 and 1.50% ofSludge 1 and Sludge 2. The solvent extract filter residue comprised0.49% of Sludge 1 and 0.71% of Sludge 2 by mass. These solids aresuspected to be calcium soaps, i.e., calcium palmitate, oleate, andstearate, which while not soluble in organic solvents at roomtemperature and pressure (Harrison 1924) may have beensolubilized under the elevated pressure and temperature of theASE extraction to later precipitate as the solvent cooled.

Elemental Analyses

The results of the 28-element ICP-OES analysis are detailedin Table 2. The relative concentrations are Ca > Na >Al > Fe > Mg > Li > Zn, with these seven elements comprising94% of the elements found within the sludge. Relatively high con-centrations of the main elements present remain within the waterafter solvent separation, showing that significant amounts of thecalcium, sodium, and lithium are not bound to the solvent solubleorganic fraction. This is important because studies on FOG by au-thors such as Nitayapat and Chitprasert (2014), He et al. (2011),Williams et al. (2012), and Keener et al. (2008) have not separatedthe water from the organics in this way and have assumed that all ofthe inorganic elements detected are inherent within the organicphase, which is not the case for this sludge.

Calcium concentrations within the organic portion of the sludgewere 135.5 and 1,303 mg=kg for Sludge 1 and Sludge 2, respec-tively. Total concentrations in the wet sludge are 932.3 mg=kg(�3.6%) for Sludge 1 and 1,973 mg=kg (�3.6%) for Sludge 2.These concentrations are only 10–20% of the total concentrationsfound by Williams et al. (2012), although those results are based ondried sludge, so could be up to 20 times lower if water content of upto 95% was included in the concentration calculation. FOG studiedby Keener et al. (2008) had a calcium content of two to four timesgreater than that of the sludge under study and a sodium concen-tration within the range found in Sludge 1 and Sludge 2.

Fourier Transform Infrared Spectroscopy

The FTIR spectra are almost identical for Sludge 1 (Fig. 4) andSludge 2 (Fig. 5), which show intense, sharp peaks at2,800–3,000 cm−1, which correspond to the C─H bond. Smallerbut still sharp peaks are seen at 1,600–1,800 cm−1, representingC═O bonds; more precisely, the peak is between 1,650 and1,750 cm−1, which relates to acids and esters. Another C─O bondis detected at 1,200–1,300 cm−1. Overall there are C─O bonds,C─H bonds, and C═O bonds, along with acids and ester bonds.

Table 1. Organic Compounds Detected during Qualitative Analysis of theSolvent Extracted Sludge by GC-MS Using an Elite 5-MS Column and theNIST Library

Compound Sludge 1 Sludge 2 Melting point (°C)

Dicyclohexylamine Yes Yes −2Palmitic acid Yes Yes 63Oleic acid Yes Yes 14Stearic acid Yes Yes 69Palmitic anhydride Yes No 61–64Pentadecane No Yes 8–10Pentadecane, 7-methyl- No Yes UnknownHexadecane No Yes 18Myristic acid No Yes 52–54Octadecane No Yes 28–30Hexadecanoic acid,2-methylpropyl ester

No Yes Unknown

Decyl oleate No Yes Unknown

Note: Melting point also shown to reference against melting test data.

Table 2. Results of the 28-Element ICP-OES Analysis of the Microwave-Digested Sludge

Analyte

Sludge 1 Sludge 2

Total(mg=kg)

Solventseparated(mg=kg)

Water(mg=kg)

Total(mg=kg)

Solventseparated(mg=kg)

Water(mg=kg)

Ca 932.3 135.5 797.9 1,973 1,302 670.4Na 303.6 96.85 206.7 77.74 33.97 43.77Mg 26.15 9.38 16.76 84.61 39.43 45.18K 13.76 4.67 9.09 25.35 11.11 14.24Al 131.63 76.91 54.71 775.5 439.57 335.96Fe 124.09 47.59 76.50 1,258 854.7 403.1Mn 1.36 0.27 1.09 22.38 13.63 8.75Ag 0.13 0.08 0.04 0.11 0.14 −0.03As 0.47 0.47 0.00 0.12 0.36 −0.23Be 4.77 1.42 3.34 6.33 1.83 4.50Ba 16.35 9.09 7.26 32.01 17.53 14.48Bi b.d. b.d. b.d. b.d. b.d. b.d.Cd 3.99 2.08 1.91 3.88 2.07 1.81Co 0.60 0.34 0.25 0.71 0.43 0.28Cr 0.28 0.13 0.15 0.65 0.39 0.27Cu 2.01 1.03 0.97 6.32 3.80 2.52Li 24.46 13.40 11.06 66.98 36.92 30.06Mo 0.69 0.32 0.37 0.73 0.40 0.33Ni 4.69 2.80 1.89 7.34 4.84 2.49Pb 3.24 1.56 1.68 8.44 4.54 3.90Sb 11.58 5.82 5.76 29.09 19.23 9.85Se 2.47 1.34 1.12 2.57 1.54 1.03Si 6.07 3.51 2.56 5.92 3.85 2.07Sr 4.75 1.53 3.22 4.27 2.47 1.80Ti 1.34 0.71 0.63 4.48 2.29 2.19Tl b.d. b.d. b.d. b.d. b.d. b.d.V 0.22 0.10 0.12 0.64 0.38 0.26Zn 18.56 11.35 7.21 108.04 69.75 38.29

Note: Units are mg=kg of wet sludge; b.d. = below detection.

Abs

orba

nce

Wave number/ cm-1

Fig. 4. FTIR spectra of unadulterated Sludge 1 in the range4,000–650 cm−1

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All of these bonds are present within palmitic and oleic acid, how-ever the most common bond in these chemicals was the C─H bond,which relates to the intense peak seen at 2,800–3,000 cm−1. Thedetection of esters confirms that not all of the oil had been degradedinto free fatty acids.

Direct comparisons with calcium palmitate, palmitic acid, oleicacid, calcium stearate, stearic acid, and the mineral oil used in themetal machining factory showed that no one chemical alonematched the sludge spectra, but each had their similarities on differ-ent peaks, except the mineral oil. Therefore it is likely that thesludge is formed of a mix of these chemicals, along with otherschemicals that were not analyzed.

Total Sludge Composition

The results from water separation, GC-MS, and ICP-OES analyseswere combined to form an overall composition for the sludge,shown in Fig. 6. The sludge has been shown to be an organicallybased inverted emulsion, composed mostly of water, oleic acid,palmitic acid, and unidentified organics, with little or no mineraloil present. These unidentified organics are suspected to be triglyc-erides (rapeseed oil or trimethylolpropane trioleate) found in thevirgin VOCF and other organic compounds that have beenqualitatively identified (Table 1).

The composition of the sludge can be described as similar to thatof FOG because water, palmitic acid, oleic acid, and calcium arepresent in similar concentrations. Water (moisture) content of thesludge is on average 32.6%, compared with 54.7% (Williams et al.2012) and 53.9% (Keener et al. 2008), although the range ofmoisture content in the studies by Williams et al. (2012), Keeneret al. (2008), and Nitayapat and Chitprasert (2014) have a widevariation of 0–95%.

Metallic Salts of Fatty Acids: Soap

Soap is defined as a salt of a fatty acid, saturated or unsaturated,containing at least eight carbon atoms or a mixture of such salts(IUPAC 2014). Soaps have been identified as a principal compo-nent of FOG (e.g., Keener et al. 2008), in addition saponificationhas been identified as a suspected causal mechanism in theformation of FOG deposits (He et al. 2011; Williams et al.2014). Thus it is important to establish whether soaps, particularlycalcium palmitate and calcium oleate, contribute significantly to thecomposition of the VOCF sludge or whether the FFAs dominate.

The mass requirements of calcium bonds to two fatty acids toform soap (Ca:FFA) from stoichiometry are 1:12.8, 1:14.1, and1:14.2 for palmitic acid, oleic acid, and stearic acid respectively.The mass requirement of sodium bonds to one fatty acid to formsoap (Na:FFA) from stoichiometry are 1:11.1, 1:12.3, and 1:12.3for palmitic acid, oleic acid, and stearic acid respectively. Assumingthat all of the calcium, sodium, and lithium are bonded to oleic acidand palmitic acid in a 1∶1 ratio, the soap content of Sludge 1 wouldbe 0.35%, and it would be 1.85% for sludge 2. Soap is a minorcontributor to the chemical makeup of the sludge compared towater and the FFAs, oleic acid, and palmitic acid. While it is fea-sible that the soap may act as a surfactant in the inverse water in oilemulsion, it is unlikely that soap is important in the formation andaccumulation of the VOCF ETP sludge.

Furthermore, the level of Ca present as calcium palmitate islower than this because calcium palmitate and calcium oleateare insoluble in water (Harrison 1924), and the water contains86% of the Ca in the sludge (Table 2). This highlights the benefitof using solvent extraction to separate water from the sludge com-pared to drying, which causes aqueous constituents to remainwithin the dried sludge and appear as part of an undifferentiatedtotal sludge assay.

Ca is present in high concentrations, which is also observed byWilliams et al. (2014) in FOG deposits. They suggested that bio-calcification may be an important mechanism in this Ca accumu-lation (and in FOG formation), however in the case of the VOCFsludge the TC, TIC, and TOC analyses show that inorganic carbon-ate such as calcite is not present in significant concentrations withinthe sludge.

The presence of FFA rather than soaps is further supported bythe melting point analysis (see comparative data in Tables 1 and 3).Calcium palmitate, calcium oleate, and calcium stearate have nodefinite melting point, rather they are observed to conglutinateat 110°C and 115–120°C, respectively, and decompose at temper-atures 140–160°C (Harrison 1924). The melting point of palmiticacid and oleic acid, principal components of the sludge, are

Abs

orba

nce

Wave number/ cm-1

Fig. 5. FTIR spectra of unadulterated Sludge 2 in the range4,000–650 cm−1

Fig. 6. Chemical breakdown of Sludge 1 (black) and Sludge 2(hatched), where 1 = water, 2 = inorganics in water, 3 = non-solvent-extractable solids including solids over 0.2 μm, 4 = solvent-extract filter residue, 5 = inorganics in solvent extract filter residue,6 = unquantified organics, 7 = palmitic acid, 8 = oleic acid, and9 = inorganics in ASE sludge

Table 3. Mean Initial and Full Melting Points of Sludge Samples: ThreeRepetitions

Sample Initial melting point (°C) Full melting point (°C)

Sludge 1 40.3 (�1.2°C) 46.0 (�0.0°C)Sludge 2 59.7 (�0.5°C) 65.0 (�0.8°C)

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reported to be 63 and 14°C, respectively, which ties in with themelting point analysis for palmitic acid and also explains the anec-dotally operator-reported thickening of the sludge in wintermonths, which on consideration of the compositional data wasprobably due to solidification of the oleic acid fraction of thesludge.

Ca is known to accumulate in cutting fluid circuits because it is aclosed loop where water is topped up to replace water lost toevaporation, which naturally leads to concentration of Ca and otherions. Ruling out Ca soaps as a major component of the VOCFsludge precludes control of Ca concentrations in the effluent, whichwas one of the early proposed strategies considered in preventingthe VOCF sludge formation.

Routes of Formation

Understanding the routes of formation is an important step in beingable to halt the formation of the floating cutting fluid–based sludge.The two main oils used at the metal-machining factory aretrimethylolpropane trioleate and rapeseed oil. Trimethylolpropanetrioleate is a triglyceride containing three oleic acids and trimethy-lolpropane as the base. The fatty acids in rapeseed oil vary, but onaverage are composed of 60% oleic acid, 4% palmitic acid, 20%linoleic acid, and 10% alpha-linoleic acid (Gunstone 2009). It issuspected that the trimethylolpropane trioleate and rapeseed oilare being microbiologically degraded to free-floating oleic acidand palmitic acid. This route is mostly biologically achieved,however low and high pH can also lead to the formation of freefatty acids. Conditions within the factory VOCF coolant circuitsare constantly monitored, with the pH being kept around 9 andthe emulsion dosed with biocide to slow biodegradation of thecutting fluids. However, during prolonged storage in the ETPstorage tanks the biocide itself will degrade or deplete, thusallowing the microorganisms to become active and biodegradethe oils.

The first stage in the degradation is bacterial lipase-promotedhydrolysis of the triglyceride (microbial rancidity) (VIU 2010).Evidence for this mechanism occurring is the qualitative detectionof palmitic acid esters and decyl oleate within Sludge 2, which werenot detected in the virgin VOCFs. During the triglyceride degrada-tion mechanism other biodegradation processes can take place, in-cluding that of the degradation of the surfactant stabilizing theemulsion, triethanolamine (West and Gonsior 1996; Speranza et al.2006). This degradation causes a decrease in the concentration ofsurfactant in the solution and decrease in pH. Degradation of sur-factant and lowering of pH reduces the stability of micelles, thusallowing the oil and its components to separate from the solution.Because the free fatty acids densities are less than that of water,they float where they agglomerate into the mass of sludge seenin the ETP storage tanks, a phenomenon known as creaming.The glycerol or trimethylolpropane is miscible in water and re-mains dissolved and is processed through the ETP, which iswhy it is not detected within the holding tank sludge. The finalstage of the sludge formation is via the generation of palmitic acidthrough breakdown of the oleic, linoleic, and alpha-linoleic acids. Itwas determined that the concentration of palmitic acid found withinthe sludge could not solely come from the fatty acid separation ofthe rapeseed ester, which contains on average only 4% palmiticacid (Gunstone 2009), therefore it must come from the degradationof the unsaturated fatty acids. The first stage in the process of fattyacid degradation starts by the breaking of double bonds, which maybe why there is no linoleic or alpha-linoleic acid found within thesludge (although alpha-linoleic, linoleic, and oleic acid have almostidentical retention times on the GC-MS columns). Once all of the

double bonds have been (bio)hydrogenated, then no further biohy-drogenation can occur, which leaves stearic acid. Stearic acid, alsoknown as octadecanoic acid, is then broken down to palmitic acid.Myristic acid, which is the next breakdown product of palmiticacid, was detected during the qualitative analyses of the sludge.

The buildup of the floating sludge on the surface of the waterand the subsequent exclusion of water from its physical structureresults in stabilization of the sludge because the microbial lipasescan no longer interact with the fatty acids and degrade them further(Gurr et al. 2002); this has important implications because in thesludge state rapid biodegradation dramatically slows, which iscounter to the intended full biodegradability of the VOCF products.Limitation of biodegradation due to sludge flotation was also notedby Hwu et al. (1998); other factors that may be important in pre-venting rapid breakdown of the sludge include lack of nutrients andhigh oleic acid concentrations (Pereira et al. 2002).

Conclusions

This study has examined in detail a floating sludge at an ETP of ametal-machining factory that has switched from MOCFs toVOCFs, from which the following conclusions are drawn.

Methods have been developed and presented for the physico-chemical characterization of VOCF sludge from a metal-machiningplant effluent. The sludge under study has been shown to becomposed of water, oleic acid, and palmitic acid, which togetherform more than 70% of the mass of the sludge. Using a liquid-liquid separation using DCM provides a useful way to separatewater content which preserves the identity of waterborne constitu-ents. Not only have the data contributed to the understanding ofVOCF sludge, but they may also prove useful in analysis and in-terpretation of similar FOG deposits.

A notable difference from FOG in the literature is that based onthe analyses only a minor constituent of the sludge can beaccounted for by Ca bonded fatty acids (soaps).

The route of formation of the sludge is thought to be through thecreaming of existing FFA in the quiescent conditions of the holdingtanks after attenuation of surfactant and biologically inducedhydrolysis and oxidation of other FFAs to palmitic acid.

These findings have significant implications for the metal-machining industry (and other lubricant-using industries) becauseglobally the switch from MOCFs to VOCFs continues. With thischange in oils comes (1) the risk of formation of the sludge in ETPswith the consequent risk of closure due to reduction in capacity tostore and treat effluent, high sludge disposal costs, storage risks,and increased costs associated with cleaning and maintenance,and (2) the risk to the environment posed by a stabilized floatingsludge that is resistant to biodegradation.

Acknowledgments

We would like to thank Cardiff University and Innovate U.K. forthe financial support for this project.

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